Analyte binding assays are useful for detecting a variety of analytes in research and clinical laboratories. Immunoassays, in many different formats, have been used to detect analytes such as viral and bacterial antigens, immunoglobulins, hormones, cell subtypes, pharmaceuticals, drugs of abuse, and toxins. Immunoassays involve the formation of a complex between an antigenic substance and an antibody. Typically, either a component of the complex or a competing antigen is labeled with either a radioisotope or a fluorescent label to allow quantitative or qualitative analyte detection. Although the sensitivity of immunoassays has been enhanced by enzyme-mediated amplification schemes, wherein a secondary signal-generating enzyme system is coupled to an enzyme/antibody conjugate, further increases in sensitivity would enhance the ability to detect analytes.
Nucleic acid hybridization assays have been used to detect specific nucleic acid analytes. Nucleic acid hybridization assays involve the formation of a complex between a target nucleic acid and a complementary probe nucleic acid. Typically, the probe nucleic acid is labeled with either a radioisotope or a fluorescent label to allow quantitative or qualitative detection of the target nucleic acid analyte. The sensitivity of nucleic acid hybridization assays has been enhanced through the use of a probe nucleic acid having a polymeric tail that is bound by a secondary signal-generating probe (see e.g., U.S. Pat. No. 4,882,269). The sensitivity of nucleic acid hybridization assays also has been enhanced by procedures that incorporate additional sites for attachment of a signal-emitting secondary probe to the hybridized probe nucleic acid (see e.g., WO 89/03891 and European Patent Application 204510). Sensitivity, however, is limited by the nature of the labels. Thus, a need exists for a more sensitive method for detecting analytes.